One problem with wind power generators is noise caused by blades. Recently, the size of wind power generators has been increased with the growing demand for high-power wind power generators. Accordingly, the rotor diameter, that is, the length of the wind turbine blades, has increased.
The longer the blades are, the faster the blade tips move. Accordingly, a larger amount of noise occurs, and a further noise reduction has been demanded.
The major sources of noise caused by blades are noise due to vortices at the tips of the blades and noise due to turbulence boundary layers occurring on the surfaces of the blades.
In the related art, to reduce the noise level, it is thought to be effective to reduce the rotational speed of a rotor. That is, if the rotational speed of the rotor is reduced, the flow speed of air to the blades is decreased, so that aerodynamic sound can be reduced. However, if the rotational speed of the rotor is reduced, the power generation efficiency is decreased.
To reduce noise without reducing the rotational speed of the rotor, for example, as disclosed in Patent Citation 1, a technique called serration has been proposed.
This is intended to suppress Karman vortices from a trailing edge of a blade by forming a serrated portion at the trailing edge so as to cause longitudinal vortices. By suppressing Karman vortices, noise is reduced.
Patent Citation 1:
Japanese Unexamined Patent Application, Publication No. 2003-336572